Thermoplastic vulcanizates (TPVs) are a class of polymer materials comprising a continuous phase (matrix) usually of partially crystalline thermoplastic polymer (such as polypropylene and propylene copolymers) and a cross-linked (“vulcanized”) disperse phase, usually elastomeric. TPVs were developed to close the gap between thermoplastic elastomers and thermoset rubbers. Having rubber-like properties (soft touch, resilience) and being processable by methods used for thermoplastic, these materials have better cost-performance ratios than traditional thermoset rubbers in certain applications.
A process specially designed to produce TPVs is dynamic vulcanization (hereinafter, briefly, DV). DV is based on cross-linking of disperse phase material during intensive shear mixing, thus preventing it to form a continuous phase. This process also generally includes phase inversion. Being usually a majority in the initial blend, the vulcanizable material forms a continuous phase, while the second component forms a disperse phase. However, the vulcanizing process leads to an increase in the viscosity of the vulcanized polymer, while the viscosity of non-cross-linked components remains unchanged or even slightly decreases. When the viscosity of the cross-linked phase becomes significantly higher than that of the said second component, the vulcanized phase begins to break into droplets, that decrease in size as the viscosity increases. At this stage, phase inversion occurs, i.e., the non-cross-linked component changes from a disperse phase to a continuous matrix, while the vulcanized component changing from a matrix to a disperse phase. The phase inversion allows to obtain TPVs with well-defined phase structure, even at very high cross-linked phase contents (70% and higher).
DV processes may be implemented as batch (using batch mixers such as Banbury) or as continuous (using compounding equipment, e.g., twin-screw extruders). The continuous process has obvious advantages (higher output, better product uniformity, lower labor costs), but the residence time in the processing equipment is significantly shorter, requiring highly efficient cross-linking system to complete the reaction inside said equipment and to prevent post-curing during the following shaping and use of the resulting products.
It should be noted that it is not required that a matrix polymer (by which expression is meant the non-cross linked component that constitutes the matrix after phase inversion has occurred) of a TPVs be absolutely insensitive to the condition in which the elastomeric phase is vulcanized to form TPVs. Some reaction between the matrix and the vulcanization agents may occur, and may even be a significant reaction (see, e.g., U.S. Pat. No. 4,183,876).
U.S. Pat. No. 4,130,534 describes TPVs compositions, called, however, “elastomeric compositions”, comprising crystalline polyolefins and butyl or halobutyl rubber, made by DV, e.g., in extruder, or not. Curing agents are listed at col. 7, lines 20-42.
U.S. Pat. No. 4,183,876 is more general than the two previous patents. It uses DV, though not necessarily, and mentions many possible alternatives, but the claims are limited to polyalkenamer rubber, defined as “random noncrystalline, rubbery polymer of cycloalkene”. Dynamic vulcanization is described as “masticating” the blend of components, e.g. in extruders, “at a temperature sufficient to effect cross-linking formation”.
U.S. Pat. No. 5,066,700 discloses a process comprising making a mixture of 1) PP or PET-PP; 2) ethylene-propylene elastomeric copolymer or diene elastomeric terpolymer; 3) peroxide; 4) furan derivative; masticating while heating, and adding oil.
U.S. Pat. No. 4,803,244 discloses cross-linking the rubber component by hydrosilylation, viz. adding a silicon hydride across a multiple bond, often with a transition catalyst. An example is rhodium-catalyzed hydrosilylation of EPDM rubber in blend with PP.
U.S. Pat. No. 6,147,160 improves on U.S. Pat. No. 4,803,244 by a) suggesting specific copolymers of, e.g., isobutylene and divinylbenzene; b) hydrosilylating them with Pt catalyst.
U.S. Pat. No. 6,331,595 discloses grafting a monomer which may be a vinyl trialkoxy silane, onto a polyolefin, using as a catalyst an organic peroxide coated onto a carrier polymer.
U.S. Pat. No. 6,448,343 discloses using amino silane in the preparation of TPV by DV whereas the disperse phase component is grafted by acid or anhydride moiety prior to the reaction with said amino silane. Among the drawbacks are a long residence time, and a low gel content.
US20030114604 relates to use of acid moieties to catalyze moisture cross-linking of silane grafted polymers, but does not relate to DV. The cross-linking stage in this case is rather slow and should take place outside the processing machine.
WO 00/68278 discloses a process for making TPVs, using ultra low density polyethylene (ULDPE) as disperse phase polymer, and crystalline thermoplastic polyolefin as matrix, while ULDPE is grafted with organo silane and then dynamically cross-linked by basic aqueous buffer solution in the presence of said matrix polymer. However, since the moisture cross-linking is rather slow, a combination of two extruders is needed, along with huge injections of aqueous solutions for achieving a reasonable gel content and cross-link density.
While the literature on TPVs is very extensive, no fully satisfactory process is available in the art. DV is preferable to other systems, but there are difficulties in carrying out full vulcanization of the elastomers used. Prior art DV processes involve significant disadvantages. In some cases phenolic resins are used in this reaction, and they are toxic. In other processes, tin dichloride is used and results in hygroscopicity of the TPVs. Some processes lead to discoloration or staining of the finished product and it is very difficult to obtain a product having a natural white color. In other processes, the elastomers have poor UV resistance. Finally, prior art processes do not permit the use of a wide range of matrices and cross-linkable components.
It is therefore the purpose of this invention to provide a process that is free from all the aforesaid disadvantages and that permits the use of a wide range of polymers.
It is another purpose to provide a process that is easily carried out and uses standard equipment, such as extruders, and that can be implemented continuously.
It is a further purpose to provide a process which causes substantially full cross-linking of the elastomer within a short residence time and which permits to use low contents of cross-linking agents.
It is a still further purpose to provide a final TPV product that has a combination of physical properties that will be set forth hereinafter.
Other purposes and advantages of the invention will appear as the description proceeds.